(1) Field of the Invention
The present invention relates to a polarization plane maintaining optical fiber, and more particularly to both a circular optical fiber having an intense function to transmit optical waves by the use of a one-directional polarization, i.e., to maintain a polarization plane and a method of fabricating the same.
(2) Description of the Prior Art
In accordance with the progress in development of fibers optical fiber, it has been being developed to practice an optical circuit such as an optical isolator or an optical switching circuit by the use of an optical integrated circuit. The waveguide structure to be used in the optical integrated circuit is basically of the slab type, which is strengthened from the necessity that a light has to be polarized so as to practice the integrated circuit by the switching circuit. It is desired to effectively couple such optical integrated circuit to another optical device by means of a circular optical fiber. In this case, it is required that the optical fiber can maintain the polarization plane in a predetermined direction.
Moreover, it has been proposed to conduct a variety of measurements by making use of the polarization plane of the light. In order to provide those measurements, the transmission of the light while maintaining its polarization plane raises a problem to be solved. It is especially desired in view of the transmission efficiency in optical energy and the consideration of fabrication that the light be transmitted, while having its polarization plane maintained, by the use of a circular waveguide tube (i.e., the optical fiber) which has reached the practical stage at present.
As the circular optical fiber maintaining the polarization plane, there has been proposed according to the prior art an optical fiber which is constructed of a core, a cladding and a jacket and in which the jacket has such an irregular thickness as to induce a stress in the core so that an index difference (which will be referred to as a "strain birefringence") may be made equal to or higher than a predetermined level to maintain the polarization plane in the core in accordance with the difference in the mechanical stresses taken in the waveguide regions and along the transverse directions intersecting at a right angle (Reference should be made to "Birefringence in elliptically clad borosilicate singlemode fibers. APPLIED OPTICS/Vol. 18. No. 2415. Dec. 1979).
In order to maintain the polarization plane, specifically, the maintenance of polarization is intensified to a greater extent as the value L (which will be called a "coupling length") to be defined by the following Equation is decreased the smaller: ##EQU1## wherein the difference in the propagation constants between two orthogonally polarized fundamental modes is designated at .alpha..beta.. If the core has a circular shape, therefore, the difference .DELTA..beta. in the above-defined constants is determined by the difference .DELTA.n in the refractive indexes for the two directions of polarization. Moreover, that difference .DELTA.n in the refractive indexes is proportional to the difference in the strain in those two directions, which is caused by the difference in the thermal expansion coefficients between the jacket and the cladding.
In the proposal thus far described, however, as means for making the thickness of the jacket irregular, the jacket has to be deformed at an initial step so that its fabricating process is complicated. In reality, moreover, the coupling length L obtained is at least 10 mm.
We, the Inventors, have realized a polarization plane maintaining optical fiber (which is disclosed in Japanese Patent Application Nos. 55-1330, entitled "Polarization Plane Maintaining Singlemode Optical Fiber"), which is constructed of the aforementioned jacket, cladding and core, by adding B.sub.2 O.sub.3 in a quantity equal to or more than a predetermined value to the cladding composed mainly of silicon dioxide and by slightly modifying the optical fiber fabricating method of the prior art.
However, since the glass having the B.sub.2 O.sub.3 added thereto fails to have its relative index difference 0.7% more than the SiO.sub.2 glass, the microbending loss is enlarged to invite another problem that it is impossible to provide an optical fiber of low loss. Moreover, in case the boric oxide (B.sub.2 O.sub.3) contained in the cladding is too much, the absorption due to lattice vibrations for a longer wavelenth (e.g., longer than 1.2 .mu.m) invites still another problem that the loss cannot be lower than 1 dB/Km for a wavelength which is intrinsically deemed as a low-loss region for the optical fiber of boro-silicate glass.